docs: Clarify Group's sizing semantics
docs: Clarify Group's sizing semantics

The documentation for ClutterGroup behaviour when setting an explicit
size is not accurate - or, actually, it was accurate by the time
ClutterGroup was first written but has been neglected in the following
release cycles.

To avoid confusion for new users of Clutter the documentation should be
slightly expanded, mentioning the exact semantics of ClutterGroup with
regards to: preferred size, explicitly set size and how to constrain the
visible area of a ClutterGroup to an explicitly set size.

Based on a patch by: Neil Roberts <neil@linux.intel.com>

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